ADP-glucose pyrophosphorylase (AGPase) is a highly regulated enzyme in the starch biosynthesis pathway. In planta the enzyme has a complex structure, an α2β2 heterotetramer, that when expressed in the endosperm of seeds is heat labile. The heat lability of AGPase is often linked to grain loss during hot weather, and therefore increasing the thermo-stability of this enzyme is of great agronomical importance. Previously, heat stabile variants of this enzyme have been transformed into maize, potato, wheat, and rice and yield increases in seed number or weight were obtained.
Conventional directed evolution methods for enhancing protein thermo-stabilities rely on collections of random mutations spread across the entire linear sequence of amino acids. In principle, such a strategy offers the chance to explore the complete sequence space of a given protein and thereby provide the globally optimal solution. In practice, however, the number of sequence variants at the DNA level (64517 for the maize endosperm large subunit) means that the required library sizes vastly outstrip the ability of molecular biology to provide them and practical screening methods to evaluate them. This necessarily means that sequence space exploration will be only partial.